linux-kernel - Re: [PATCH 2/3] s390/vfio-ap: introduce two new r/w locks to replace wait_queue_head

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Message-ID: <20210611174150.GX1002214@nvidia.com>
Date:   Fri, 11 Jun 2021 14:41:50 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, borntraeger@...ibm.com,
        cohuck@...hat.com, pasic@...ux.vnet.ibm.com, jjherne@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        frankja@...ux.ibm.com, imbrenda@...ux.ibm.com, hca@...ux.ibm.com
Subject: Re: [PATCH 2/3] s390/vfio-ap: introduce two new r/w locks to replace
 wait_queue_head_t

On Fri, Jun 11, 2021 at 07:11:50PM +0200, David Hildenbrand wrote:

> > Also, not convinced down_write_trylock() is appropriate from a sysfs
> > callback, it should block and wait, surely? Otherwise userspace gets
> > random racy failures depending on what the kernel is doing??
> 
> It might we worth exploring lock_device_hotplug_sysfs() which does a
> 
> "return restart_syscall()" with some delay.

The ideal design from a sysfs should be a single
down_write_killable().

restart_syscall will just create a weird spinlock that is hopefully
unlikely to spin :\

Jason